Magnetic resonance imaging (MRI) is a medical imaging modality that can create pictures of the inside of a human body without using x-rays or other ionizing radiation. MRI uses a superconducting magnet to create a strong, uniform, static magnetic field. When a human body, or part of a human body, is placed in the magnetic field, the nuclear spins associated with the hydrogen nuclei in tissue water become polarized, wherein the magnetic moments associated with these spins become preferentially aligned along the direction of the magnetic field, resulting in a small net tissue magnetization along that axis. MRI systems also include gradient coils that produce smaller amplitude, spatially-varying magnetic fields with orthogonal axes to spatially encode the MR signal by creating a signature resonance frequency at each location in the body. Radio frequency (RF) coils are then used to create pulses of RF energy at or near the resonance frequency of the hydrogen nuclei, which add energy to the nuclear spin system. As the nuclear spins relax back to their rest energy state, they release the absorbed energy in the form of an RF signal. This signal is detected by the MRI system and is transformed into an image using a computer and known reconstruction algorithms.
As mentioned, RF coils are used in MRI systems to transmit RF excitation signals and to receive MR signals, the RF signals emitted by an imaging subject. Coil interfacing cables may be used to transmit signals between the RF coils and other aspects of the processing system, for example to control the RF coils and/or to receive information from the RF coils. The coil interfacing cables may be disposed within the bore of the MRI system and subjected to electro-magnetic fields produced and used by the MRI system. The cables may support transmitter-driven common mode currents which create field distortions and/or unpredictable heating of components. These field distortions may result in a shadow of the cables appearing within an image reconstructed from received MR signals.
Conventionally, baluns or common mode traps that provide high common mode impedances may be utilized to mitigate the effect of transmitter-driven currents. However, placing the common mode traps or blocking circuits at appropriate locations may be difficult, as the appropriate placement may vary based on the positioning of a cable or coil associated with the common mode traps. Also, excessive voltage and/or power dissipation may occur even if conventional common mode traps or blocking circuits are placed at appropriate locations.
Further, baluns or common mode traps positioned too close to each other on a cable may become coupled due to fringe magnetic fields, thereby resulting in a detuning of the baluns which may adversely affect the functioning of the baluns.